In large pot, bring water and salt to boil over medium high heat. Add rind pieces and boil until tender, about 5 minutes. Strain. Transfer rinds to a large metal bowl.

In saucepan, combine sugar, cider vinegar, peppercorns, cloves, pickling spice and gingerroot. Bring to a boil over medium high heat, stirring until sugar dissolves. Simmer for 15 minutes, until slightly reduced. Pour over watermelon rinds in bowl. Place plate over top to keep rinds submerged in liquid. Cover and refrigerate for one day. Transfer to a glass jar and keep sealed in the refrigerator for up to 2 weeks.

Source:

Watermelon.org